Background: Advertisement is key revenue generator of the website
Premise: company gets money each time user clicks on the add
Premise: applications used to block the adds are gaining popularity among users of website.
Conclusion: To prevent from future loss, website must ban from running any such application on website.
(A) At least some users who currently do not use the mentioned applications will start using them in the future.
This is having negative effect.
(B) People using such applications would not have clicked on any of the advertisements on the website even if they were not using the applications.
This too have negative effect. If they do not click now, they are likely not contributing to loss in future.
(C) Changing the algorithm that displays the advertisements on the website will not make it impossible for applications to selectively filter out advertisements without blocking the entire website.
Try negating This assumption, it suggest that if it was possible to just change algorithm and not block website then they will not go for banning the application. So This is potential assumption.
(D) Most of the users of drench.com do not find the information displayed in the advertisements useful.
This has no effect
(E) Customers who use the advertisement-blocking applications on drench.com also use them on websites that are competitors of drench.com.
This has no effect.
So C is correct assumption